The gauge does not need to be incredibly accurate, but it does need to be consistent, and precise. You can get the precision from a digital gauge and they are very cheap. I believe they are also consistent as long as you press down well enough to fully open the valve, and that there is nothing leaking out around the valve when taking the measurement. That at least will remove much of the measurement error. It's the only thing I can think of that would cause all four tires to show that much of a drop since I know it's cold, but I don't think the temperature in Toronto dropped 60 degrees C in a few days. lol

Hope that helps at least some!

The gauge does not need to be incredibly accurate, but it does need to be consistent, and precise. You can get the precision from a digital gauge and they are very cheap. I believe they are also consistent as long as you press down well enough to fully open the valve, and that there is nothing leaking out around the valve when taking the measurement. That at least will remove much of the measurement error.

If your 20 minutes of driving was at highway speeds for most of that, you might have gotten the tires 10 deg C warmer than when they have sat overnight. But if you were just driving around town at relatively low speeds, I would be surprised if you got more than a few degrees C. That's especially true if you're driving through snow since the snow will cool the tires quickly and keep your speeds low.

One thing you can try, while it's still really cold outside, fill the tires to 35 or 36 PSI when they are still cold from sitting overnight. Then if it gets significantly warmer, and the tires are warm from driving, you are still under the max pressure, and not so low that they are under inflated when it's really cold (since you filled them to 36 when it was really cold).

Hope that helps at least some!